In the manufacture of photographic products, several distinct layers may be coated simultaneously, with the number of individual layers ranging from two to as many as ten or more. Typically, a multilayer photographic coating may consist of sensitized layers and/or additional non-imaging layers. These solutions may have vastly different makeups and hence vastly different properties. Most commonly, aqueous gelatin solutions are used as a binder or suspending medium for the constituents of a given layer. The concentration of gelatin, polymers, thickening agents, and other solids determines the viscosity of the solution. It is highly desirable to be able to manipulate the viscosity of such solutions as they are handled, delivered, and then coated since optimum viscosities during the different operations may differ.
For example, photographic coatings require hardeners. Such coatings are very dilute and have a very low viscosity. To make a good coating, higher viscosities are preferred, usually 50 centipoise or higher. The advantages of high viscosities during coating include reduced flow after coating due to air impingement on the mobile (non-set) coating, reduced flow after coating due to non-planar base, reduced interfacial wave growth due to interfacial strain rate discontinuities, increased coating uniformity and the like. To insure that the required viscosity is achieved, thickeners are generally included in photographic emulsions during their preparation which is well upstream of the apparatus whereby the emulsion is coated onto the base.
But high viscosities create significant handling problems and require very high pump capability. Further, as the solubility limits of the dissolved ingredients in very high viscosity solutions are approached, the solids begin to come out of solution. When that happens, the settling solids cause coating streaks, the physical and/or sensitometric properties of the solution become unbalanced, filters become plugged, and other problems well known in the art arise.
In addition, high viscosity solutions present significant delivery problems. In order to coat uniformly and without imperfection, a coating solution must be delivered through a series of devices designed to improve the quality of the final coated film such as flowmeters, de-bubblers, mixers, filters, the coating hopper and the like. A pressure head is required to move a coating solution through each of these devices. The pressure head may be induced by gravity (elevation) or in-line pumps. Since pressure drop and solution viscosity are proportional for flow through a circular tube (or any other delivery expedient having a geometry where laminar flow exists), as solution viscosity increases, the pressure drop required to deliver the solution also increases if a constant flowrate is to be maintained. In practice, most delivery systems are operated very close to the upper pressure drop limit of the system. Consequently, any increase in solution viscosity could require a significant capital investment to deal with the increase.